Drag occurs on the external surface of air vehicle engines due to friction, as is frequently encountered when the air vehicle is operated in a level flight mode. In addition, drag occurs on the external surface of air vehicle engines due to boundary layer separation. Under certain circumstances, an initial laminar fluid boundary layer flow over a surface may transition to a turbulent flow or a boundary layer separation flow, which results in an appreciable increase in surface drag. Boundary layer separation may occur when the air vehicle is operated at high angles of attack or when localized air speed adjacent the leading edge of the engine is transonic.
Boundary layer separation drag may be especially problematic for commercial aircraft when, for example, the aircraft is in climb maneuver and an engine fails or experiences engine burn-out. In such a circumstance, the aircraft may undergo significant drag induced moments, which may result in loss of aircraft control.
Previous boundary layer separation drag reduction efforts include engine exterior surface contouring. Such surface contouring, however, must accommodate specific aerodynamic requirements of various flight operating regimes (e.g., take-off, climb, level flight, engine burn-out) and flight velocities. Such accommodations result in inefficient drag reduction over all flight conditions. In addition, surface contouring results in significant increases in manufacturing costs and air vehicle weight penalties.
Other boundary layer separation drag reduction efforts include air suction devices which suck air into the engine surface in order to reduce the occurrence of the boundary layer separation phenomenon. Such an approach, however, necessarily employs suction pumps with associated weight, space and cost penalties.
Accordingly, it is desirable to design an engine drag reduction system which is relatively low cost, is able to achieve drag reduction across a range of flight operating conditions, does not increase drag under any flight operating conditions and does not substantially increase the weight of the aircraft.
In addition, conventional noise reduction schemes have focused at the inner surface of the engine. Conventional schemes allow intake engine air to enter the inner surface of the engine through a plurality of small pores. Each pore corresponds to a cavity or cell. Air is not allowed to flow through the cell, but rather becomes trapped in the cell. Thus, the conventional noise reduction scheme is a no-flow system. The geometry and dimensions of each cell is a function of sound wave lengths associated with undesirable sounds. It is desirable to design an drag reduction system which could integrate existing noise reduction designs.